I. Field of the Invention
The present invention pertains to hydrogen storage powders and methods for making the same. More particularly, the present invention pertains to a method for making an electrochemical hydrogen storage powder with an engineered surface oxide to protect the powder from catching fire and to provide easier activation.
II. Description of the Related Art
Hydrogen storage materials have found use in various technologies, including battery electrode materials, fuel cells, getters, heat pumps, and the storage of hydrogen gas. Hydrogen storage materials are materials capable of absorbing and desorbing hydrogen respectively. Examples of some hydrogen storage materials include: metallic elements, such as Mg, Ti, V, Nb, Pd, and La, and some intermetallic alloys, such as TiFe, Mg2Ni, MgNi, misch metal based AB5, and Laves phase based AB2. It is generally practiced when making hydrogen storage materials to form the hydrogen storage material into a porous substructure to take advantage of high surface reaction area. A porous substructure is typically formed by powderizing a hydrogen storage material, forming the powderized material into a composite body with a suitable substrate material and subjecting the composite body to one or more pretreatment steps or xe2x80x98activationxe2x80x99. Formation of a powdered material followed by pretreatment increases surface area, active sites, and porosity among other characteristics of the hydrogen storage material.
Powderized hydrogen storage material is particularly useful in the formation of battery electrodes. Methods for making powderized hydrogen storage materials and battery electrodes therefrom are generally known and have been described in a number of patents, including for example, U.S. Pat. Nos. 4,716,088; 4,670,214; 4,765,598; 4,820,481; and 4,915,898, the disclosures of which are herein incorporated by reference. Methods for making metal hydride battery electrodes typically include applying and fixing a pulverized hydrogen storage material to a conductive substrate. A number of additives may also be added to the powder to increase cohesion to the conductive substrate and to improve battery performance. Such additives may include binders and conductive filler.
Hydrogen storage materials may be powderized by a number of methods. The particular method to be used depends upon the material""s composition, hardness, tendency to form surface oxides, and desired end use. Known powderization techniques include mechanical and chemical methods of pulverization and combinations thereof, including machining, milling, shooting, granulization, atomizing, condensation, reduction, chemical precipitation, or electrodeposition. Additionally or alternatively, other conventional size reduction techniques may be used, including, abrasion, shearing, ball milling, hammer milling, shredders, fluid energy, and disk attrition. To obtain useful particle size distributions or classification, pulverization is typically followed by sieving. Other techniques which do not necessarily require pulverization involve rapid quench methods such as jet casting and gas atomization.
While certain size reduction techniques work well for some materials, there is no single technique that works for all materials. For instance some materials are soft enough to crush using only mechanical crushing methods. Some materials are best suited for a combination of mechanical and chemical pulverization methods. Other alloys are too hard for mechanical crushing and must be pulverized by chemical methods. As such, many mechanical sizing techniques are not effective for pulverizing very hard materials, particularly those having a Rockwell hardness of greater than 45.
Bulk hydrogen storage material is typically formed by melting various metals together and casting them in the form of a button or ingot. Bulk hydrogen storage material may be compositionally and structurally ordered, disordered or anywhere in between. As reported in A Nickel Metal Hydride Battery for Electric Vehicles, by S. R. Ovshinsky, M. A. Fetcenko and J. Ross, Science, Vol. 260, Apr. 9, 1993: xe2x80x9cAmong the elements that have become available for alloy formation in disordered electrode materials are Li, C, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Sn, La, W, and Re. The list contains elements that can increase the number of hydrogen atoms stored per metal atom (Mg, Ti, V, Zr, Nb, and La). Other elements allow the adjustment of the metal-hydrogen bond strength (V, Mn, and Zr) or provide catalytic properties to ensure sufficient charge and discharge reaction rates and gas recombination (Al, Mn, Co, Fe, and Ni); or impart desirable surface properties such as oxidation and corrosion resistance, improved porosity, and ionic conductivity (Cr, Mo and W). The wide range of physical properties that can be produced in these alloys allows the MH battery performance to be optimized.xe2x80x9d
Order and Disorder form a spectrum and may be engineered into hydrogen storage materials to improve the above mentioned characteristics and others. The amount of structural and compositional order may be both material and process dependent. For example, more highly ordered materials may be formed by a conventional melt-and-cast. A slow cooling process allows crystal growth and substantial structural order to take place depending on the chemical formula. Highly disordered materials, on the other hand, are typically formed by melting with rapid quench, fast cooling methods. Rapid cooling provides a more highly disordered material, also dependant on the chemical constituents.
One successful method of pulverizing hydrogen storage alloy using hydrogen pulverization was developed at Ovonic Battery, Inc., the method of which is disclosed in U.S. Pat. No. 4,893,756, issued Jan. 16, 1990 to Fetcenko et al., and is entitled xe2x80x9cHydride Reactor Apparatus for Hydrogen Commination of Metal Hydride Hydrogen Storage Materialxe2x80x9d, the disclosure of which is herein incorporated by reference. Through continuous and ongoing research endeavors, Ovonic Battery, Inc. developed the first hydrogen storage reactor for pulverizing hard, hydrogen storage materials for use in negative electrodes for batteries. The method has paved the way for the development of new materials with improved characteristics for the formation of battery electrodes.
According to Fetcenko et al., hydrogen storage powders may be made from a bulk material, such as a metal ingot or alloy by hydride/dehydride cycling comminution. The hydride/dehydride process reduces a metal hydride or hydrogen storage material from a large ingot or bulk size to particles. To accomplish the aforementioned comminution, bulk material is placed in a stationary hydrogen reactor. The reactor is placed under vacuum and purged with argon to remove any residual air. Hydrogen is then back-filled into the reaction chamber to a pressure of at least 25 psi. The hydrogen is absorbed in the hydrogen storage material, which thereby causes volumetric expansion of the metal lattices to comminute the hydrogen storage material. Hydrogenation may take several hours and may reach elevated temperatures. Cooling is provided to maintain the reaction vessel at a temperature of less than about 100xc2x0 C. After hydrogenation, the comminuted material is dehydrogenated to remove hydrogen from the hydrogen storage material. The material may then be packaged under an oxygen free atmosphere for later formation into a battery electrode.
The Fetcenko patent and its progenies teach methods of hydrogen pulverization wherein the hydrogenation step is pressure controlled. Unfortunately, a pressure-controlled reaction may result in excessive heating of the hydrogen storage material. Excessive heating is undesirable in that it may cause unpredictable changes to the compositional and structural order or disorder of the hydrogen storage material. Additionally, excessive heating results in a long cooling period and increased production time.
Exposure of hydrogen storage materials to air can produce unpredictable and variable oxide formation on a significant portion of the particles. Uncontrolled or uneven oxidation of hydrogen storage materials typically results in an undesirably thick, dense and non-uniform oxide formation on the surface of the material, which makes activativation difficult. The resulting surface conditions render the surface unusable for immediate use as a negative electrode material for a battery. For example, a thick oxide layer hinders initial activation of the electrode by acting as an insulative barrier. An insulative barrier can interfere with electrochemical kinetics and transport processes that take place at the metal surface/electrolyte interface. Thus, manufacturing techniques for producing hydrogen storage powders with little or no oxide formation, such as packaging the powder under an oxygen free atmosphere, have been used. Yet, without an oxide surface layer, the hydrogen storage material is extremely pyrophoric due to the high heat of formation of metals such as La, Ce, Ti, V, Zr, Al with oxygen. Pyrophoric powders can react violently when exposed to air. Thus, pyrophoric materials pose a significant fire hazard in large scale manufacturing plants where safe working conditions are essential.
A number of methods have been evoked to overcome some of the problems associated with oxide formation on the hydrogen storage material. These methods focus almost exclusively on post powder formation by employing pretreatment steps or xe2x80x98activationxe2x80x99. Activation, as used herein, specifically refers to treatment methods used to improve hydrogen transfer rate in an electrochemical hydrogen storage material after powder formation. For example, see U.S. Pat. No. 4,716,088, issued Dec. 29, 1987 to Reichman et al., which teaches a method for activating a rechargeable hydrogen storage, negative electrode, the disclosure of which is herein incorporated by reference.
Pretreatment methods work to activate the hydrogen storage material by improving, for example, surface area, porosity, and catalytic activity. Pretreatment methods primarily include electrical formation and etching. Electrical formation is defined as charge/discharge cycling required to bring the battery electrode up to its ultimate performance when inserted in a battery. For a number of alloys, electrical formation is essential for maximum battery performance at both high and low discharge rates. For instance, certain prior developed ViZrNiCrMn alloys require as many as 32 cycles of charge and discharge at various rates to fully activate an electric battery. It has been reported that this electrical formation causes expansion and contraction of the negative electrode alloy material as it alternately stores and releases hydrogen. This expansion and contraction induces stress and forms in-situ cracks within the alloy material. The cracking increases the surface area, lattice defects and porosity of the alloy material. Heretofore, NiMH battery electrodes have required this electrical formation step.
Electrical formation is material dependant, as different active hydrogen storage materials are prepared by various methods under distinct conditions, and formed into electrodes by several methods require different pretreatment steps. Hence, although no detailed method of electrical formation suitable for all electrodes can be described, electrical formation generally involves a relatively complex procedure of cycling the prepared battery through a number of charge/discharge cycles at varying rates and to varying depths.
Etching is also used to pretreat battery electrodes and is also material dependant. Etching, as defined herein, is chemical/thermal activation of the battery electrode material when treated with an etchant. Etching typically involves a relatively lengthy period of immersing the electrode in a concentrated alkaline or acidic solution. The degree and ease of etching is highly dependant on the chemical formula or composition of the material to be etched and the stability of the surface oxide. For example, alkaline solutions used to etch battery electrode materials include concentrated potassium hydroxide, sodium hydroxide, etc. Many etching methods require that the immersion take place at elevated temperatures. Additionally, etching often requires a long residence time. Although residence time depends on temperature and concentration, it is typically on the order of hours to a few days.
At present, in situ activation of electrodes in the battery is impractical due to the extreme conditions required for activation. For example, one problem of in situ activation is that separators used within batteries are sensitive to the elevated temperatures of greater that 60xc2x0 C., which are normally used for activation. Another problem is that conventional etching typically requires a high concentration of KOH where batteries tend to have a much lower concentration of KOH. Thus, present day activation is not preformed in situ.
The impact of pretreatment or activation on manufacturing decisions can particularly be seen between the commercial advantages of two popular battery electrode materials, Laves based AB2 and misch metal based AB5 materials. AB2 and AB5 hydrogen storage alloys are. generally known. AB2 materials include various Ti, V, Ni, Cr, Mg and Zr alloys belonging to the laves phase of intermetallic compounds and generally having a hexagonally symmetric C14 and cubically symmetric C15 structures. AB5 alloys differ in chemistry, microstructure and electrochemistry from AB2 alloys. AB5 materials include, for example, such alloys where A is selected from the group consisting of rare earth elements and B is selected from the group consisting of Ni, Cu, Co, Fe, Al, Si and Mn.
A number of differences exist between AB2 and AB5 hydrogen storage materials that make AB2 alloys a preferred battery electrode material. A atoms in AB2 alloys (Ti, Zr) have a lower atomic weight compared with the heavy rare earth A atoms of AB5 alloys. A lower atomic weight results in AB2 alloys having a higher gravimetric energy density than AB5 alloys. There are also structural differences between AB2 and AB5 alloys. For example, AB2 alloys have two basic structures (C14 and C15), while AB5 materials have one basic structure (CaCu5). The multiple structures of AB2 alloys allow for a higher degree of obtainable disorder as compared with AB5 alloys. The advantages of AB2 over AB5 alloys makes AB2 materials a significantly better battery electrode material when higher capacity, energy density, rate, and cell life are compared.
Despite the fact that AB2 alloys provide a superior battery electrode material, AB5 materials tend to require less activation. Activation has been a long and persistent problem to commercial battery manufacturers. In particular, activation encumbers the manufacture with investments in specialized capital equipment in the form of battery chargers along with increased labor and utility cost for additional processing steps. These costs are significant and are typically passed on to consumers in the form of increased battery costs. On the other hand, some manufactures tend to favor battery materials, such as AB5 alloys, that require less activation. Unfortunately, such materials, as explained above, tend to be inferior electrochemical, hydrogen storage materials in terms of hydrogen storage capacity, cycle life and increased power. As such, some manufacturers sacrifice the performance provided by AB2 alloys by opting to use AB5 alloys to increase manufacturing cycle time by reducing activation requirements. In either case, higher costs are passed on to consumers either in the form of inferior products that need more frequent replacement or in the form of higher production costs associated with activation.
As the demand for better batteries at lower costs continues, manufactures will seek to improve the quality of their products with reduced manufacturing cycle time but without reducing the safety of their operations. Therefore, there is presently a need for an electrochemical, hydrogen storage alloy having high capacity, energy density, rate, and cell life and a method for making the same, which can be safely manufactured on a large scale, but requires minimal activation.
The present invention, to address the above mentioned deficiencies and others provides a particalized, anti-pyrophoric hydrogen storage. material having enhanced initial activation and a method for making the same. The method provides a unique surface or surface layer upon the particalized material, which requires reduced activation when formed into an electrochemical hydrogen storage electrode.
In a preferred aspect hereof, the present invention provides a process for making an AB2, anti-pyrophoric, electrochemical hydrogen storage powder with a unique, oxidized surface layer having enhanced initial activation.
The present invention also provides a method for making a comminuted, hydrogen storage alloy having an average particle size of 100 xcexcm or less from a high hardness, bulk hydrogen storage material without the need for mechanical pulverization.
Generally, the method of the present invention includes providing a particalized hydrogen storage alloy and oxidizing the particalized alloy with a controlled oxidation reaction. The method preferably includes agitating the particalized alloy during the oxidizing step. The method may begin by providing a bulk hydrogen storage material, then comminuting the bulk hydrogen storage material by hydrogen pulverization, dehydriding the comminuted material, and oxidizing the comminuted material with a controlled oxidation reaction to form an anti-pyrophoric, electrochemical hydrogen storage alloy having an average particle size of 100 xcexcm or less without the need for mechanical pulverization.
In another aspect hereof the method includes a further step of adding a passivating material to the particalized alloy before oxidizing the mixture with a controlled oxidation reaction. The passivating material preferably becomes dispersed about the surface of the oxidized particles to form a protecting surface layer that reduces surface oxidation to enhance initial activation.
In still another aspect of the present invention, the method comprises a step of forming a hydrogen storage material with a sacrificial modifier, such as Al, Sn, or Co or a combination thereof, prior to powderizing the hydrogen storage material. The sacrificial modifier provides a hydrogen storage powder having enhanced initial activation.
For a more complete understanding of the present invention, reference is made to the following detailed description and accompanying drawings.